Fix Debugbar url tail slash issue

[vibbow]

Description
In toolbarloader.js.php, the tail slash in url is trimmed, so when build a request url in xhttp.open, it should add one tail slash (Otherwise server will do a 301 redirection to the one with tail slash first)

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide